This invention relates to the measurement of oxygen saturation level of hemoglobin in arterial blood and more particularly to a non-invasive oximetry sensor.
Non-invasive oximeter commonly take advantage of the difference in the light absorption coefficient of hemoglobin and hemoglobin oxide with respect to light in the red and infrared ranges. This type of oximeter normally includes sensors that are placed against patient tissue which is well perfused and include sources for emitting light at one or more wavelengths into the tissue and a light detector for detecting the amount of light which passes through the tissue. The amount of light absorbed at each wavelength is used to calculate oxygen saturation in the patient's blood in accordance with Lambert-Beer's law. Such sensors are normally placed on the fingertip, earlobe, nasal septum, or forehead of the patient and preferably include means for retaining the sensor in position for the extended periods during which such measurements are made. Notwithstanding the requirement for durability, the probes are preferable disposable so as to insure sterility.
One type of oximeter sensor includes an adhesive strip for holding the LEDs and detector against the patient's tissue. Mother type includes a spring-biased clamp for attaching the sensor in position. The use of these probes involves considerable expense because the relatively costly LED's are integrally mounted in the disposable portion of the probe. In addition, while these methods of attachment have been effective for retaining the LEDs and detector in position, they tend to become uncomfortable if worn for extended period.
The Lambert-Beer's law for calculating blood oximetry employs coefficients which are dependent upon the wavelength of the light being emitted by the specific LEDs in the sensor. However, due to manufacturing tolerances, the wavelengths light emitted by LEDs used in oximetry sensors varies widely about some nominal value. Therefore, the accuracy of the oximetry measurements may suffer greatly unless the LEDs are carefully sorted to insure wavelengths within a narrow band width or the processor is reprogrammed for each successive oximeter sensor. However, reprogramming is impractical since the sensors are discarded after each use. One solution to these problems is disclosed in U.S. Pat. No. 4,700,208 and involves sorting the LEDs into narrow band-width ranges and providing a coding resistor in each sensor which advises the processor the specific wavelengths of the particular LEDs in that sensor. The processor then determines the appropriate coefficients to De used in the oximeter calculation from a look-up table. While this latter method has been commercially successful, the requirement for sorting and the need for a coding resistor increase the cost of manufacturing the sensors.